Kuz’mich, Valeriy, et al. “FORMATION OF THE CONCEPT OF ANGLE BY MEANS OF METRIC GEOMETRY ON GEOMETRIC MATERIAL OF 9TH GRADE”. Physical and Mathematical Education, vol. 29, no. 3, June 2021, pp. 6-12, https://doi.org/10.31110/2413-1571-2021-029-3-001.